1. Field of the Invention
The present invention relates to a method for manufacture of a semiconductor device and, more particularly, a semiconductor device wherein a T-shaped gate (mushroom-like gate) is formed on a substrate of a compound semiconductor or the like.
2. Description of Related Art
In any very high speed transistor known as HEMT or the like, it is necessary that the gate be formed so as to be sectionally T-shaped (mushroom-like) for reducing the gate resistance while shortening the length.
FIGS. 8A through 8F are sectional views illustrating successive steps executed sequentially in a first conventional example of a method for forming such a T-shaped gate.
[A] As shown in FIG. 8A, a first resist layer b and a second resist layer c are sequentially formed on a substrate a which is composed of a compound semiconductor. PA1 [B] Then, as shown in FIG. 8B, the second resist layer c is processed by exposure to an electron beam of a relatively low energy. The reason for employing such a low-energy electron beam is so as to protect the first resist layer from exposure to the electron beam. An exposed portion d of the second resist layer c is shown. PA1 [C] Subsequently, as shown in FIG. 8C, the first resist layer b is exposed by the use of a relatively high energy. An exposed portion e of the first resist layer b is illustrated. The area of the exposed portion e is considerably smaller than that of the exposed portion d of the second resist layer c. PA1 [D] Thereafter, as shown in FIG. 8D, the second resist layer c and the first resist layer b are patterned by a developing process. PA1 [E] Next, as shown in FIG. 8E, a film is formed by evaporating a suitable gate material such as aluminum, whereby a T-shaped gate g is formed in the aperture of the first resist layer b. PA1 [F] Finally, as shown in FIG. 8F, the first resist layer b and the second resist layer c are removed together with the aluminum film f on the second resist layer c. PA1 [A] After a first resist layer b is formed on a substrate a, the first resist layer b is exposed to an electron beam and then is developed to form an aperture h therein. PA1 [B] Subsequently, a film is formed by evaporation of a suitable gate material (e.g., aluminum), and the first resist layer b is lifted off to consequently form a gate f as shown in FIG. 10B. In this stage, however, the gate has no portion which corresponds to the head of a mushroom. PA1 [C] Then, as shown in FIG. 10C, a SiN film i and a second resist layer c are sequentially formed on the substrate a. PA1 [D] Next, the second resist layer c is exposed by the use of an electron beam and then is developed to form an aperture j in a portion around the gate f, as shown in FIG. 10D. This aperture is greater than the aforementioned aperture h. PA1 [E] Subsequently, as shown in FIG. 10E, a gate material film is formed by evaporation so as to form the head of the T-shaped gate.
The steps of forming the T-shaped gate g are thus completed by the above procedure.
FIG. 9 is a sectional view of a second conventional example achieved by partially modifying the T-shaped gate forming method illustrated in FIG. 8.
According to this example of the T-shaped gate forming method a first resist layer b and a second resist layer c are sequentially formed on a substrate a, and the material of the first resist layer is selected so as to be lower in sensitivity than that of the second resist layer. As illustrated in FIG. 9, the two resist layers b and c are exposed in a single process. An aperture is formed in the lower or first resist layer which is smaller than an aperture in the upper or second resist layer c. According to this conventional method, a single exposure step is sufficient to attain this purpose since the respective sensitivities of the first resist layer b and the second resist layer c are different from each other.
FIGS. 10A through 10E are sectional views illustrating successive steps sequentially in a third conventional example for forming the T-shaped gate.
Thereafter the layers are lifted off as in the first conventional example of FIG. 8, whereby merely the T-shaped gate alone is left on the substrate a.
In the conventional T-shaped gate forming methods illustrated in FIGS. 8 and 9, there are some disadvantages such as variations of size result of the post-development aperture due to variations in the thickness and the sensitivity of the first resist layer b and the second resist layer c, and satisfactory reproducibility cannot be attained with respect to the dimensions and the sectional contour of the T-shaped gate. Also, there exists a problem of insufficient lift-off facility.
For another conventional T-shaped gate forming method such as illustrated in FIG. 10, it is necessary to execute the exposure step twice by the use of an electron beam which requires a considerable time for tracing the path, hence resulting in a disadvantage of low throughput. Also, the problem of deficient lift-off facility still remains.
As described above, in the entire conventional examples of the T-shaped gate forming methods illustrated in FIGS. 8 through 10, there are common disadvantages of insufficient lift-off facility and low yield rate and difficulty to form a satisfactory T-shaped gate.